A tenth planet

by Wm. Robert Johnston
last updated 4 March 2007

Note added 14 September 2006: The International Astronomical Union has named 2003 UB313 and its satellite: Eris and Dysnomia, respectively. The Minor Planet Center has assigned minor planet numbers to 2002 UB313 and Pluto as well: (136199) and (134340), respectively.

Note added 24 August 2006: The International Astronomical Union has passed a resolution classifying Pluto and 2003 UB313 as "dwarf planets". This new category also includes the asteroid (1) Ceres, and a minimum of 12 other trans-Neptunian objects/asteroids are candidates for this transitional classification. For more see this page.

On 29 July 2005 came announcement of the discovery of a trans-Neptunian object larger than Pluto. The issue of whether the astronomical community will officially designate it a planet has yet to be determined--but provisionally, based on size, I'll refer to it here as the tenth planet in our solar system.

On 28 July 2005, J. L. Ortiz et al. announced discovery of an object nearly as large as Pluto, now designated 2003 EL61. Apparently the object had been previously observed by a team led by Brown, who was withholding announcement of this and two other large trans-Neptunian objects (TNOs) until a conference in September. Discovery credit in astronomy is based on who announces first, which was the Ortiz team. Reportedly someone accessed one or more computers of the Brown team, prompting them to announce discovery of the other two large TNOs on 29 July. One of these, now designated 2003 UB313, is clearly larger than Pluto based on its observed brightness. The other, designated 2005 FY9, is over half the diameter of Pluto. Additionally, the Brown team reported having observed a satellite of 2003 EL61, constraining mass of the system to about 30% that of Pluto. A second satellite of 2003 EL61 was discovered a short time later.

The new planet "Eris", appears to be between Pluto and the Earth's Moon in size. It is probably mostly ice (water ice and frozen methane). It takes about 560 years to orbit the Sun once in its very eccentric orbit. At its closest to the Sun, it is just inside Pluto's average distance from the Sun; at its furthest, it is three times as far from the Sun as Neptune. Its orbit is inclined 44° to the ecliptic, the plane of the Earth's orbit. All the planets out to Neptune orbit close to the ecliptic, and the high inclination of 2003 UB313 is part of why it had not been discovered sooner.

On 1 October 2005, M. Brown and his team announced discovery of a satellite to 2003 UB313. The satellite is estimated to be about 350 km across.

To add to these finds, two more satellites of Pluto were discovered, both estimated at on the order of 100 km across and both orbiting outside the orbit of Charon.

The first measurement of the diameter of 2003 UB313 was reported in February 2006. The team of Bertoldi, Altenhoff, Weiss, Menten, and Thum made thermal infrared measurements from the IRAM telescope in Spain and found a diameter of 3000 km plus or minus 400 km. In April the team of Brown, Schaller, Roe, Rabiowitz, and Trujillo reported results from direct imagery of 2003 UB313, indicating a diameter of 2400 km plus or minus 100 km.

The table below lists information on Neptune, Pluto, the newly discovered "planet" temporarily designated 2003 UB313, and the other largest known TNOs.